A general permanent-magnet-embedded motor includes a stator iron core, and a rotor which is arranged on an inside diameter side of the stator iron core and in which a permanent magnet is embedded in an inner part of a rotor iron core. The rotor iron core is formed by stamping of an electromagnetic steel sheet into a predetermined shape and by swaging and laminating of a plurality of stamped electromagnetic steel sheets. In the rotor iron core, magnet insertion holes are formed at substantially equal intervals in a circumferential direction according to the number of magnetic poles in the permanent magnet. Each magnet insertion hole is stretched in an axial direction and a permanent magnet is inserted into an inner part of each magnet insertion hole.
Here, an outer side surface in a radial direction of the permanent magnet inserted into each magnet insertion hole and an inner side surface in the radial direction thereof are connected to each other via a thin part formed between the magnetic poles of the permanent magnet. Then, magnetic flux generated in each permanent magnet passes through a core back of the stator iron core and goes back to the rotor iron core. However, a part of the magnetic flux from a surface of each permanent magnet becomes leakage flux that passes through the thin part, stays in the inner part of the rotor iron core, and goes back to the permanent magnet instead of going to the core back. That is, a part of the magnetic flux that goes out from one surface of each permanent magnet passes through the thin part and enters a different surface of the permanent magnet without passing through the stator iron core. Such leakage flux does not contribute to torque and becomes a factor of increasing an iron loss in the rotor. Thus, the leakage flux is preferably controlled as much as possible. In such a manner, since a thin part formed between the magnetic poles becomes a path of leakage flux, a width thereof is preferably as narrow as possible.
However, since centrifugal force during a high-speed rotation of the rotor of the permanent-magnet-embedded motor acts on the permanent magnet, centrifugal force that acts on each part of the rotor iron core becomes large in proportional to the square of the number of rotations in a case where an upper limit of the number of rotations of the rotor becomes higher. Thus, in the thin part of the rotor iron core, it is necessary to increase strength of a part that supports a surface on an outer side in a radial direction of the permanent magnet. For example, it is necessary to widen a width of the thin part in proportional to the square of the number of rotations.
As a method of improving the strength of the rotor iron core other than widening of the width of the thin part between the magnetic poles, it is considered to divide each of the permanent magnet, which configures each magnetic pole, and the magnet insertion hole into two or more in a circumferential direction and to provide a bridge, which connects an outer side in a radial direction of the rotor iron core and an inner side in the radial direction thereof, between the divided magnet insertion holes.
However, similarly to the above-described thin part between the magnetic poles, this bridge becomes a path of leakage flux and a width in a circumferential direction of the magnet is decreased for a width of the bridge. Thus, when the bridge is provided and the width in the circumferential direction of the magnet is decreased, there is a problem that an effective magnetic flux that contributes to torque is decreased and a size of the permanent-magnet-embedded motor is increased in order to cover the decrease.
As a method that does not depend on providing of a bridge, it is considered to use a steel sheet with mechanical strength higher than that of an electromagnetic steel sheet including silicon steel or Armco iron (hereinafter, referred to as “high-strength steel sheet”) in a rotor iron core. However, a high-strength steel sheet generally has an inferior magnetic characteristic, specifically an inferior iron loss characteristic, compared to an electromagnetic steel sheet. Thus, an iron loss in the rotor is increased greatly in a case where an electromagnetic steel sheet in the whole rotor iron core is replaced with the high-strength steel sheet. Thus, an iron core including an electromagnetic steel sheet is used in a part where the most of an iron loss in the whole rotor is generated, that is, a part on an outer side in a radial direction of a permanent magnet and an iron core including a high-strength steel sheet is used in the remaining part.
For example, in a motor described in Patent Literature 1, a rotary shaft includes an iron core part on an outer side in a radial direction of a plurality of permanent magnets (hereinafter, referred to as “A part”) and an iron core part other than the A part (hereinafter, referred to as “B part”). The B part includes an iron core part on an inner side in a radial direction of each permanent magnet and an iron core part between magnetic poles. The A part includes silicon steel or Armco iron with a superior magnetic characteristic and the B part includes carbon steel with a mechanical strength higher than that of the A part. Then, the iron core part between the magnetic poles is formed in a T-shape. Hereinafter, the T-shaped iron core part will be referred to as an “engagement part.” The engagement part is extended from an outer peripheral part of the B part toward an outer side in a radial direction between the magnetic poles and has a leading end branching in a T-shape. Moreover, at a leafing end of the part branching in the T-shape, a recessed part to be engaged with a protruded part in an end part in a circumferential direction of the A part is formed. With this configuration, coming off of the iron core part of the A part from the rotary shaft due to the centrifugal force in a rotation of the rotary shaft is prevented.